Non-metallic inclusions in steel (hereinafter referred simply to as “inclusions”) lead to, as well as causing defective or flaws of steel product, the deterioration of weldability or strength/ductility and further the deterioration of corrosion resistance and, particularly, the larger the size thereof, the more serious such adverse effects. Therefore, a number of methods are developed for reducing the number of or reforming the inclusions, and particularly large-size inclusions.
At the threshold of the development, techniques such as reforming of an oxygen contamination source such as slag, optimization of deoxidation conditions or the like, and moreover removal of inclusions by a secondary refining apparatus such as RH were rigorously developed, and these techniques are being used even now. However, since these techniques cannot meet the required performance of steel product that has been escalated, a control technique of inclusions morphology such as Ca treatment has been developed to respond to such a demand in combination with the existing techniques.
In recent years, the required performance of steel product is further escalated, and a number of new techniques have been proposed to respond to this demand.
For example, Patent Literature 1 discloses a technique for improving bore expandability by use of MgO or MgO-containing inclusions, and Patent Literature 2 discloses a technique for dispersing harmful oxygen as fine MgO by controlling the content of Mg in steel in a specific range.
The present applicant also proposes, in Patent Literature 3, a technique for reducing harmful coarse carbonitride inclusion constituents by generating carbonitrides using a Ca—Al-based oxysulfide inclusion constituent as nuclei.
In this way, the latest techniques utilize the inclusions rather than simple removal or reduction of inclusions that has been performed in the related prior art.
On the other hand, there are various types of inclusions which primarily have constituents such as sulfides, oxysulfides or carbonitrides other than oxides, singly or otherwise in combination. In the past, it was at most one or two of these types of inclusions that hinder efforts to obtain the characteristics required for steel product. For example, surface defects in a cold-rolled steel sheet are principally caused by the coarse oxide type, and the deterioration of the weldability in a structural material such as a steel beam is caused by the sulfide type, so that a desired effect could be attained by taking specific measures against specific inclusion types as described above.
In recent years, however, it has been demanded also to simultaneously satisfy a plurality of characteristics, in addition to the escalated required performance of steel product. For example, a combination of high strength and high corrosion resistance, a combination of high strength and high workability or the like is sought after.
When two kinds of characteristics, let's say, characteristic A and characteristic B, are simultaneously required, for example, two measures against the relevant inclusions such as a measure “a” for satisfying the characteristic A and a measure “b” for satisfying the characteristic B must be taken at the same time according to the conventional point of view.
However, taking a plurality of measures simultaneously may create problems in performance, besides cost and productivity.
For example, although the sulfides can be reduced by reducing the content of S in steel, the decrease in content of S can lead to increase in the number of the oxide type inclusions since the interfacial tension between molten iron and inclusions reduces according to the decrease in content of S to thereby deteriorate the floatation separability of inclusions. Further, the reduction in content of S in steel leads to a change in content of N in steel which results from an increased rate of denitrification or nitride absorption of molten iron, and as a result, the number of nitrides can likely vary.
Namely, the decrease of a specific type of inclusions can create problems such as the increase of other types of inclusions and the deterioration of inclusions controllability.
Further, when a plurality of characteristics are simultaneously required with particularly high performance, what matters is not the number of specific types of inclusions such as oxides or sulfides that affects other characteristics, but the total number of two or more types of inclusions such as oxides, sulfides, oxysulfides and carbonitrides. For example, even if MnS is reformed with Ca or the like to be made harmless for the purpose of improving the corrosion resistance of steel product, Ca-based inclusions after the reformation may degrade the surface quality of the steel product. In such a case, it is necessary to reduce the total number of inclusions after the reformation, in addition to render MnS harmless, and the necessary measures therefor are further complicated.
In this way, when a plurality of different characteristics are to be satisfied at a high level, the measures against inclusions are complicated to end up in deteriorating the stability of quality, while causing the productivity and costs of product to be deteriorated. Since this deterioration of stability causes the reduction of product yield, further efforts for commercial industrial production are needed while the supply of the product is possible.